The present invention relates to an ultraviolet detector that converts the incident amount of ultraviolet radiation into a different wavelength by a wavelength conversion element for detection.
An ultraviolet detector employing a wavelength conversion element of interest to the present invention is disclosed in, for example, Japanese Patent Laying-Open No. 64-47921. FIG. 12A is a sectional view of the detection unit disclosed in this publication, and FIG. 12B is a diagram of the entire structure of the ultraviolet detector. Referring to FIGS. 12A and 12B, the detection unit of the ultraviolet detector includes a casing 107 having an upper opening 105 provided at the upper portion of the detection unit and a lower opening 106, in which a plate-like phosphor (wavelength conversion element) 108 is provided between openings 105 and 106. An optical fiber 109 is arranged at an end face of phosphor 108. An interference filter 110 is provided in the passage of optical fiber 109. A light receiving element 104 receives the fluorescence input via optical fiber 109 and interference filter 110. In this ultraviolet detector, the incident ultraviolet through opening 105 is converted into visible light by phosphor 108 to enter optical fiber 109 from the end face. However, most of the ultraviolet is output through opening 106 without being converted.
Another ultraviolet detector of interest to the present invention is disclosed in, for example, Japanese Patent Laying-Open No. 6-317463. This publication discloses an ultraviolet detector provided with a bandpass filter and a photodetector in a direction orthogonal to the traveling direction of the ultraviolet radiation to be measured.
As a conventional ultraviolet detector employing a wavelength conversion element, Japanese Patent Laying-Open No. 5-231929 discloses the structure in which a light receiving element is arranged at the rear of a wavelength conversion element.
In the aforementioned conventional ultraviolet detectors with the structure in which the light receiving element is located at the rear of a wavelength conversion element, there was a problem that the disturbance light cut filter must be formed of a separate member in order to prevent introduction of disturbance light of high transmittance into the wavelength conversion element. There was also the problem that the light receiving element is degraded when thin in thickness since the wavelength conversion element generally has a low conversion efficiency. In order to prevent the exit of unconverted UV (ultraviolet) light, it is desirable to set the thickness of the direction in which UV light is transmitted great to allow complete conversion internally. It is also to be noted that complete conversion cannot be implemented if the incident power is great. In the conventional optical system, the light receiving element will be degraded. If the wavelength conversion element is made thick, the absorption ratio of the wavelength-converted light increases to result in lower detection sensitivity of ultraviolet radiation.
Furthermore, in the case where visible light is provided to the optical fiber via the end face of a plate-like phosphor as in the ultraviolet detector shown in FIGS. 12A and 12B, sufficient quantity of light could not be obtained. As a result, sensitivity could not be achieved.
The present invention is directed to the above problems. An object of the present invention is to provide an ultraviolet detector with reliable ultraviolet detection sensitivity and with less degradation of the light receiving element.
An ultraviolet detector of the present invention is formed of a material that converts ultraviolet into light of a different wavelength, and includes a wavelength conversion element having a first plane on which ultraviolet is incident and a second plane from which converted light is output, and a light receiving element receiving the converted light output from the second plane. In the wavelength conversion element, the first plane and the second plane are non-parallel with each other, and the dimension in the direction perpendicular to the first plane is greater than the dimension in the direction perpendicular to the second plane.
According to the present invention, the wavelength conversion element has a relatively large dimension in the travelling direction of the ultraviolet, and a relatively small dimension in the direction towards the light receiving element. Therefore, the amount of ultraviolet arriving at the light receiving element after being output from the wavelength conversion element to become disturbance light is small, and the amount of ultraviolet arriving at the light receiving element after being incident on the first plane and output from the second plate is small. Thus, degradation of the light receiving element caused by the ultraviolet is small. Also, the ratio of the converted light absorbed by the wavelength conversion element is small since the travelling distance of the converted light through the wavelength conversion element before arriving at the light receiving element is short.
Preferably, the first plane is orthogonal to the second plane. Since the light receiving element is in the direction orthogonal to the incident direction, there is the advantage of being impervious to disturbance light.
Preferably, the wavelength conversion element is plate-like, and one of the end plane is the first plane, and either the front plane or back plane sandwiching the first plane and having a region of the largest area is the second plane. It is preferable to set one of opposite end planes having the region of the smallest area as the first plane.
Further preferably, at least the second plane of the wavelength conversion element is planar with respect to the ultraviolet wavelength. By setting the side plane of the wavelength conversion element located at the light receiving element side as a mirror plane, the diffusion component at the surface can be eliminated to suppress the output of the ultraviolet or disturbance light. The second plane and the plane opposite to the second plane can be set planar with respect to the ultraviolet wavelength. All the planes adjacent to the first plane can be set planar with respect to the ultraviolet wavelength.
Further preferably, the light receiving element is arranged so that its center is located opposite to the position closer to the first plane than the center position of the second plane.
Further preferably, incident angle restriction means is further provided for restricting the incident angle of the ultraviolet entering the first plane. The incident angle restriction means restricts the incident angle of ultraviolet entering the first incident plane including the normal of the second plane to be within a predetermined angle, and allows the ultraviolet in the second incident plane perpendicular to the first incident plane to be incident at an incident angle greater than the predetermined angle.
Here, the incident angle refers to the angle between the normal of the plane of the point where the ray is incident and the ray.
The incident plane refers to the plane that includes the travelling direction of the wave incident on the surface and the perpendicular line of the surface.
Preferably, xe2x80x9cpredetermined anglexe2x80x9d corresponds to an angle at which the ultraviolet incident at an incident angle within that angle is totally reflected at the second plane and does not reach the light receiving element. As to the ultraviolet in the second incident plane, it is not necessary to intentionally restrict the incident angle since the ultraviolet radiation, even if entering at a large angle, will not reach the light receiving element assuming that the influence of irregular reflection within the casing of the ultraviolet detector is neglected. Rather, allowing incidence at a large incident angle will cause a larger amount of ultraviolet to be directed towards the wavelength conversion element to provide the advantage of increasing the quantity of converted light.
Further preferably, an incident window is provided formed of a material converting ultraviolet radiation into converted light of a different wavelength in order to attenuate the ultraviolet incident on the wavelength conversion element.
Since a wavelength conversion material is provided above the wavelength conversion element, no dent will be generated at the incident portion of the ultraviolet. Accordingly, there is less possibility of dust being gathered. The influence by the disturbance light can be reduced.
Further preferably, the ultraviolet detector further includes a detection circuit providing a detection signal of a level corresponding to the level of output of the light receiving element, display means turned on when the level of the detection signal is within a set range, and sensitivity adjustment means for adjusting the sensitivity of the detection signal with respect to the amount of ultraviolet received by the ultraviolet detector.
The sensitivity can be adjusted by altering the amplification factor of the detection circuit. Also, an aperture can be used to adjust the sensitivity capable of changing the transmitting amount of ultraviolet and restricting the amount of ultraviolet incident on the wavelength conversion element. It is preferable that the range of the display means has an upper limit value and a lower limit value.
According to another aspect of the present invention, an ultraviolet detector is formed of a material that converts ultraviolet into light of a different wavelength, and includes a wavelength conversion element having a first plane on which ultraviolet is incident and a second plane from which converted light is output, and a light receiving element receiving converted light output from the second plane. In the wavelength conversion element, the first and second planes are non-parallel with each other, and the dimension in the direction perpendicular to the first plane corresponds to the dimension that can convert at least 80% the ultraviolet incident from the first plane into converted light of a different wavelength.
According to the present invention, degradation of the light receiving element caused by ultraviolet is small since the amount of ultraviolet arriving at the light receiving element after output from the wavelength conversion element to become disturbance light is small. More preferably, the dimension in the direction perpendicular to the first plane corresponds to a dimension that can convert at least 90% the ultraviolet incident from the first plane into converted light of a different wavelength.